The present invention relates to a piston crank mechanism for a reciprocating type internal combustion engine, and more specifically to a link for a multi-link type piston crank mechanism.
Japanese Patent Application Publication No. 2004-124776 which the applicant filed shows a multi-link type piston crank mechanism for a reciprocating type internal combustion engine which connects a piston pin and a crank pin. This multi-link type piston crank mechanism includes an upper link connected with the piston pin of a piston, a lower link connecting the upper link and the crank pin of a crank shaft, and a control link having a first end pivotally supported by a body of an engine, and a second end connected with the lower link. The upper link and the lower link are rotatably connected with each other through the upper pin. The control link and the lower link are rotatably connected with each other through a control pin.
In this multi-link type piston crank mechanism, the lower link receives a combustion pressure received by the piston, from the upper pin through the upper link. The lower link is actuated like a lever so as to transmit force to the crank pin. Accordingly, it is necessary that the lower link has strength and rigidity to rotatably support the upper pin, the control pin and the crank pin, and to hold relative positions of the upper pin, the control pin and the crank pin, even when the lower link receives the input from the upper pin, the control pin and the crank pin.
On the other hand, it is necessary to ensure facility of assembly operation of the lower link to the crank shaft. In the above-mentioned Patent Application Publication, the lower link has a two part structure divided into two parts (a lower link upper section and a lower link lower section) along a dividing surface which passes through a center of a crank pin bearing section. The lower link upper section and the lower link lower section are tightened with each other by a plurality of bolts. For example, the plurality of the bolts are inserted from the below (that is, from the lower link lower section's side), and screwed into an internal thread in the lower link upper section.